1. Field of Invention
The present invention generally relates to a method for making a conductive electrode for a composite polymer material, and more particularly, relates to a method for making a metallic electrode on which a microrough surface is formed by a composite plating process.
2. Related Art
For the demands of electrical conductivity, abrasion resistance and wiring with other metal elements, a conductive polymer composite is generally coated with a metallic film so as to be made into a reactive element, such as a capacitor or a resistor.
Due to the different coefficients of expansion for the metallic electrode and the conductive polymer, poor contact between the two generally occur. A first problem is that when the polymer composites are applied to resistors or components of positive temperature coefficient, the poor contact between the metallic electrode and the conductive polymer becomes more critical since they encounter cyclic or non-cyclic temperature variances. The second problem concerns with the resistance of interface between the metallic electrode and the conductive polymer. Since the conductive polymer is made from conductive grains and insulating polymer material, during the adhering process of the metallic electrode to the conductive polymer, the insulating polymer material may flow into the interface of the conductive grains and the metallic electrode, causes higher resistance and decreases the performance of the finished polymer composite.
FIGS. 1a to 1c are sectional views showing the interface of the conductive polymer 10 and the metallic electrode 12 in three stages of enlargement. The views clearly show that the contact surfaces of the polymer 10 and the metallic electrode 12 are smooth.
To solve the problems of poor contact and higher interface resistance, U.S. Pat. Nos. 4,689,475 and 4,800,253 disclose compositions of metallic electrode and conductive polymer element in which the metallic electrode has a microrough surface, for example, having irregularities which protrude from the surface with a height of 0.1 to 100 microns, to improve its adhesion to the conductive polymer. As shown in FIGS. 2a to 2c, the three stage enlarged sectional views of the interface of the conductive polymer 10 and the metallic electrode 14, the microrough surface of the metallic electrode 14 will improve the adhesive strength of the electrode 14 to the polymer 10, and enhance the conductivity of the electrode 14 and the conductive polymer 10 by the larger contact area of the microrough surface. But, the processes for forming the microrough surfaces are not disclosed in the two patents, and the roughness of the electrode 14 is not enough. In the U.S. Pat. No. 4,689,475, the metallic electrode 14 and the polymer 10 contact only by impacting. The U.S. Pat. No. 4,800,253, which is a continuation-in-part of U.S. Pat. No. 4,689,475, makes the electrode in direct physical contact with the conductive polymer element and having two-stage protrusions which may enhance the connection of the electrode and the polymer. But, during impacting, there is no excess cavity for the polymer composite to flow, as a result, the insulating polymer material flowing into the interface of the conductive grains and the metallic electrode still decreases the conductivity of the electrode and the polymer.